Ukrainian President Viktor Yanukovych will return to work on Monday after taking time off due to an acute respiratory infection, the presidency said in a statement on Sunday.

"The president of Ukraine is feeling well, his health is satisfactory. Viktor Yanukovych intends to return to work on Monday," it said.

Ukraine's opposition on Sunday called for international mediation and appealed for Western financial aid for the first time in their protests against President Viktor Yanukovych.

As tens of thousands of people rallied in Kiev, boxer-turned-politician Vitali Klitschko told the crowd he had requested "international mediation in our negotiations with Yanukovych so there are no misunderstandings."

Ukrainian Foreign Minister Leonid Kozhara on Saturday dismissed the injuries sustained by a protester found caked in blood as "a scratch", as the man recovered in hospital in intensive care.

"Physically this man is in a good condition. The only thing he has is a scratch on one of his cheeks," Kozhara told Al-Jazeera television on the sidelines of the Munich Security Conference.

The daughter of jailed Ukrainian opposition leader Yulia Tymoshenko on Saturday said attempts to negotiate with the authorities were doomed to failure, as she pressed for the West to take a harder line.

Yevgeniya Tymoshenko spoke to Agence France Presse in Kiev in a phone interview from the sidelines of the Munich Security Conference in Germany where she met with EU Enlargement Commissioner Stefan Fuele and Italian Foreign Minister Emma Bonino.

Ukrainian authorities on Saturday accused opposition protesters of "torturing" a police officer in the basement of an occupied public building in Kiev as an opposition politician looked on.

The interior ministry said protesters in the capital Kiev on Friday night captured a policeman who wore plain clothes while on duty.

The party of jailed former prime minister Yulia Tymoshenko said on Saturday that the Ukrainian authorities were considering the introduction of a state of emergency in a bid to break up the protest movement against the government.

On Friday, Ukraine's SBU security service announced a criminal investigation into what it said was an opposition attempt to seize power.

The United States and EU traded unusually sharp barbs with Russia Saturday over Ukraine's future amid concerns that Kiev could resort to possible military intervention to end anti-government protests.

Neither side pulled any punches, with U.S. Secretary of State John Kerry saying what happens in Ukraine is crucial for Europe's future while his Russian counterpart Sergei Lavrov blasted willful and two-faced Western interference.

Ukraine's opposition warned Saturday the military may move against anti-government demonstrators, ahead of talks with U.S. Secretary of State John Kerry over the country's worst crisis since independence.

The warning came hours after the army weighed in on the crisis for the first time, calling on President Viktor Yanukovych to act urgently to end the turmoil.

The White House said Friday it was "appalled" at indications a leading Ukrainian opposition protester, who surfaced after being missing for eight days, had been tortured.

Washington was "deeply concerned" by increasing reports of protesters disappearing and being beaten and of attacks on journalists during Ukraine's deepening political crisis -- as well as by suggestions that President Viktor Yanukovych's security forces were involved, said White House spokesman Jay Carney.

U.S. Secretary of State John Kerry said Friday that measures pledged by Ukraine's embattled leader to address protesters' demands did not go far enough.

"The offers... have not yet reached an adequate level of reform," he told reporters after talks in Berlin with his German counterpart, Frank-Walter Steinmeier.